1. Field of the Invention
This invention relates to the boresight correlation and non-uniformity compensation that must occur before target handover from the FLIR to the missile seeker, and more specifically to a method of correlating un-NUCed missile video to the FLIR and an improved scene based method of non-uniformity compensation.
2. Description of the Related Art
In guided missile systems, certain calibrations must be performed before target handover from the target designation device such as a forward looking infrared (FLIR) to the missile seeker. These include performing a boresight correlation to correct for any imprecise mechanic alignments between the FLIR and missile seeker and performing a non-uniformity compensation to reduce pixel-to-pixel response non-uniformities in the seeker's IR imager. These adjustments improve the missile's chance of acquiring and then tracking the target to impact.
As shown in FIGS. 1 and 2, a soldier 10 uses a shoulder launched missile system 12 to target and launch a missile 14 to destroy a tank 16. The missile system is provided with a forward looking infrared (FLIR) system 18 that includes a high resolution imager that acquires a high resolution image 20 of the scene within its FOV 22. The missile 14 is outfitted with a missile seeker 24 having a low resolution imager that acquires a low resolution image 26 of the scene within its FOV 28. The FLIR and missile seeker communicate information over a channel such as a data bus 30. As illustrated, the FLIR and seeker's FOVs are not aligned and the non-uniformity of the IR imager is such as to almost completely obscure the tank 16 within the low resolution image 26.
Automated systems of performing boresight correlation have been in the field for decades, e.g. LANTIRN. Substantial knowledge has been acquired concerning robust approaches, for example, edge pre-filtering and masked bi-level correlation for textured scenes, with spot-feature based correlation for bland scenes. However, these methods require a relatively uniform response across the missile seeker image. In a ground-to-ground missile, image uniformity is achieved by completely cooling down and non-uniformity compensating the seeker's IR imager before boresight correlation begins. Once all three serial operations are completed and the data handed over to the missile seeker, the soldier will see seeker video with minimal noise and centered on his FLIR video. This requires time, which exposes the soldier to enemy fire and endangers the mission of destroying the target.
Although non-uniformity compensation (“NUCing”) is required to provide images of sufficient quality to perform boresight correlation, its primary function is to reduce the fixed-pattern noise level of missile IR video for acquisition by the soldier and possibly during tracking post-launch. NUC is required because the response of the IR imager on a pixel by pixel basis can change dramatically over time and based on environmental and operating conditions. The NUC procedure should be effective at removing or at least substantially reducing the pixel-to-pixel variability in the response of the imager, should be done quickly and should require minimal operator involvement (“knob tweaking”).
The predominant approach for NUCing uses a blurred version of the scene created optically, through motion of the IR seeker, or through temporal averaging. The premise is that on-average all pixels should see the same value. Based on this assumption, any high frequency components that are detected in the blurred image for each pixel are deemed to be the result of non-uniform pixel response. The blurred image is corrected to remove the high frequency components. The same correction is then applied to the subsequent non-blurred image. This approach is serviceable for relatively “flat” imagery but struggles with scenes which contain significant content at high spatial frequencies. These may be perceived as non-uniformities and “compensated” producing scene and body-motion dependent artifacts.
Once NUCing is complete and missile video with a low enough noise level is available, boresight correlation can be performed to correct any misalignment between the missile and FLIR. Boresight correlation is typically performed by first resampling the FLIR image to a coarser resolution to match the seeker's resolution. Next, the NUCed missile image is sent across the data bus and correlated against the re-sampled FLIR image. The correlation process is well known to those skilled in the art. The position of the correlation peak indicates the position of the center of the missile image relative to the FLIR's boresight (center of the FLIR's field of view). Once this relative boresight offset is known, the target position and size from the FLIR can then be mapped into the missile image.
To improve the effectiveness of these weapons systems and to protect the soldiers, techniques for simplifying and streamlining the boresight correlation are needed. Furthermore, techniques for NUC that provide higher bandwidth, improved noise reduction and minimum user involvement that is applicable to severely non-uniform scenes would further improve the efficacy of the weapons system and other IR imaging systems.